1. Field of the Invention
The present invention relates to an engine auxiliary unit driving equipment for a vehicle such as a passenger car and a truck, in which engine driving force is transmitted from a crank pulley fixed to an engine crankshaft, via a belt, to each driven pulley of a plurality of engine auxiliary units.
2. Description of Related Art
It is a recent tendency that a driving torque for operating engine auxiliary units is higher as each of the engine auxiliary units is required to generate a larger output. For example, an alternator for a vehicle is required to have a higher power generation capacity since electric loads, typically, due to various safety control devices are newly demanded.
To cope with the requirement of the driving torque increase, a poly V pulley having a multi number of V shaped grooves and a poly V belt having a multi number of V shaped projections, which contact each other with larger contact areas, have been widely used for driving the respective engine auxiliary units without a slip between the pulley and the belt. To transmit higher driving torque, which is highly demanded recently, it is contemplated to more increase the contact areas of the pulley and the belt in such a manner that a winding or overlapping angle at which the pulley and the belt come in contact with each other is enlarged or a piece number of grooves of the pulley and a piece number of projections of the belt are increased.
However, a space of an engine room is getting smaller to achieve a slant nose shape of a vehicle body, which reduces a cruising resistance of the vehicle and brings a lower fuel consumption, or to secure a larger passenger room space. Accordingly, a freedom of layout of the respective engine auxiliary units in the engine room is restricted so that it is very difficult for each of the pulleys and the belt to be arranged so as to have more enlarged winding angle. On the other hand, the pulley and belt whose respective piece numbers of grooves and projections are more increased have been put into use even in a compact passenger car, as exemplified by an alternator disclosed in JP-Y2-6-6688 in which a poly-V pulley thereof has six or more pieces of grooves.
Further, an inertia force of each rotor of the engine auxiliary units, which is fixed to the driven pulley and rotatable together therewith, becomes larger. Accordingly, at a time when an engine revolution largely varies due to an explosion stroke of the engine, typically, at an idling time when the engine revolution is low and unstable, the inertia force of each rotor of the engine auxiliary units is operative to increase a speed of the belt, which is to be reduced by a speed reduction of the crank pulley.
This will cause to fluctuate tension of the belt in a longitudinal direction and to flap the belt. When a tension fluctuation frequency is substantially coincident with a natural frequency that is defined by a distance between pulleys, a mass per unit length of the belt and so on, a resonance is likely to occur, causing the larger belt flapping that results in enhancing belt noises and shortening life time of the belt. In particular, in the alternator that is one of the various engine auxiliary units, whose entire body has been enlarged to meet the demand of larger electric load, a weight of a rotor coil has been gained to produce a larger magnetic force, resulting in increasing the inertia of the rotor. Accordingly, the larger belt flapping, or the resonance, is likely to occur more frequently in the belt around the pulley of the alternator.
To solve this drawback, an alternator that is provided with a clutch pulley having a one-way clutch mechanism is proposed, as disclosed in JP-A-2000-130563. With this construction, the clutch mechanism is locked at an engine acceleration so that the engine torque is transmitted to the alternator but is raced at an engine deceleration so that the inertia of the rotor of the alternator is not transmitted to the belt.
Further, the pulley is attached to each of the engine auxiliary units with a so-called ‘overhang’ arrangement in which the belt tension is applied to a position away from each main body of the engine auxiliary units. Accordingly, a moment load applied to the pulley causes to incline the pulley in an exerting direction of the belt tension. Therefore, the belt that puts on the respective pulleys rotates in such a bevel shape that a round length of the belt on a width side away from each main body of the engine auxiliary units is shorter than that on a width side near thereto, as shown in FIG. 9. As the piece number of grooves of the poly-V-pulley increases, the bevel shape becomes more distinctly slanted one.
Since the round length of the belt on one width side becomes different from that on the other width side, tensile strength produced in a width direction as shown by arrows in FIG. 9 act on the belt. If the belt tension in a longitudinal direction is increased to lower the belt slip, the tensile strength produced in a width direction becomes larger. As a plurality of core wires extending in a longitudinal direction are embedded in the belt, the belt has a sufficient resistance against tensile strength acting in the longitudinal direction. However, the belt does not have a sufficient resistance against the tensile strength acting in a width direction so that there likely occurs a crack in the belt due to the tensile strength in a width direction, resulting in shortening a lifetime of the belt.
Moreover, the clutch pulley disclosed in JP-A-2000-130563 has an advantage that the tension fluctuation is limited and the flapping of the poly-V belt is restricted since the inertia of the rotor is not transmitted to the poly-V belt. However, the clutch pulley is composed of many parts such as rollers, springs and bearings and the manufacturing processes including heat treatment and assembly processes are complicated so that the clutch pulley can not be manufactured at a lower cost.